I've gone to 13.3gn BD with CCI 300s, various 155gn bullets, and a COAL of 1.260". With the 150gn Noslers, I've gone to 13.0gn, but could go a bit further. I would recommend something in the 12.5-12.9gn range for what you want. Just make sure to hand weigh, and work up safely.

Anything 12.8gn and up will make big, pretty fireballs.
Like this:
<------------------------